1. Field of the Invention
The present invention relates to a charger for charging or discharging a recording medium for forming an image in electrostatic electrophotography. More particularly, the present invention relates to a charger for uniformly charging a surface of a recording medium by corona discharge in an image forming apparatus such as a copying machine, a printer, etc., and having a discharging electrode having a plurality of projections in the direction of a rotational axis of the recording medium.
2. Description of the Related Art
As is well known, an image forming apparatus using an electrostatic electrophotographic system is constructed by processing sections of charge, exposure, development, transfer, separation, cleaning and discharge. Namely, in a process for recording an image, a charger uniformly charges a surface of an image carrier as a recording medium formed on a conductive supporting body composed of e.g., an aluminum drum with respect to a rotated photoconductive layer. An optical image of an original image is next exposed onto the charged surface of the image carrier through an optical exposure device so that an electrostatic latent image according to this optical image is recorded on this carrier surface. Subsequently, toner is electrostatically attached to the electrostatic latent image on this image carrier and is then developed so that a toner image is formed on the image carrier surface. The toner image on the image carrier is then transferred onto a transfer material by a transfer device and is fixed by a fixing heater. Residual transfer toner left on the image carrier surface is removed therefrom by a cleaner and is collected in a predetermined collecting section. Residual charges are removed by a discharger from the image carrier surface after the cleaning operation to perform the next image forming operation.
For example, the recording medium as the image carrier is constructed by a photosensitive body in which an organic photo conductor (OPC) as the photoconductive layer is formed on the conductive drum. A corona discharger is generally used as a charger for providing charges for a surface of this recording medium in many cases.
In one corona discharger, a very thin conductive wire is covered with a conductive shield plate in a peripheral portion except for a shield portion opposite to the recording medium. A high voltage is applied to the wire so that corona discharge is caused to provide charges for charging by an electric current flowing through the recording medium. In another charger using corona discharge, a saw-toothed discharging electrode having many sharp projections arranged in line is disposed instead of the wire for corona discharge. A charging operation of this charger is performed by corona discharge from the sharp projections.
For example, the charger using the above saw-toothed discharging electrode (which is called a saw-toothed electrode in the following description) is proposed in specifications of the following patents.
(1) U.S. Pat. No. 4,591,713 (corresponding to Japanese Patent Application Laying Open (KOKAI) No. 60-158582)
(2) U.S. Pat. No. 4,725,731 (corresponding to Japanese Patent Application Laying Open (KOKAI) No. 63-14176)
(3) U.S. Pat. No. 4,725,732 (corresponding to Japanese Patent Application Laying Open (KOKAI) No. 63-15272)
(4) U.S. Pat. No. 3,691,373 (there is no corresponding Japanese Patent Application)
(5) British Pat. No. 1,388,084 (corresponding to Japanese Patent Application Laying Open (KOKAI) No. 49-8241)
In particular, charging characteristics are slightly influenced by a shape and an operating state of a discharging portion of each of saw-toothed electrodes at a forming stage thereof.
For example, when a saw-toothed electrode described in the above British Pat. No. 1,388,084 is formed, teeth of this electrode are formed by grinding in a manufacturing technique of the saw-toothed electrode having a zigzag shape. Accordingly, the charger is very expensive and it is difficult to stabilize dispersion in shape of the saw-toothed electrode.
In a charger described in the above U.S. Pat. No. 3,691,373, the point angle of a saw-toothed electrode is set to be steep when a discharging electrode is formed. Accordingly, great dispersion in height of the saw-toothed electrode having such a steep point angle is caused when a plurality of electrodes are formed in line.
In another general charger, distances between tip portions of saw-toothed electrodes and a grid electrode are different from each other. Therefore, impedances between the tip portions of the saw-toothed electrodes and the grid electrode are different from each other. Accordingly, discharging actions of the respective tip portions are different from each other on a recording medium surface so that no discharging operation can be uniformly performed. As a result, charging irregularities on the recording medium surface are caused.
A method for increasing a total electric current flowing through the saw-toothed electrodes is considered as a simple improving method for reducing these charging irregularities. However, when the total electric current is increased, a voltage applied to the saw-toothed electrodes is increased. A discharging electric current is increased when the voltage applied to the saw-toothed electrodes is increased. Therefore, an amount of ozone generated from a discharging portion is increased so that an image carrier surface is influenced by this ozone, thereby reducing the quality of an original image.
When the amount of ozone is increased, this ozone is bonded to various gases and foreign materials in the air floating within an image forming apparatus so that nitrogen oxides (NO.sub.X), silicon oxides (SiO.sub.2), etc. are generated. These oxides are attached onto surfaces of the saw-toothed electrodes and the grid electrode so that discharging ability of the saw-toothed electrodes and ability for controlling a charging potential of the grid electrode are reduced.
Further, it is necessary to prevent leak discharge from the tip portions of the saw-toothed electrodes to other unnecessary portions by an increase in applied voltage by increasing the total electric current. To prevent this leak discharge, it is necessary to excessively secure distances from discharging portions of the saw-toothed electrodes to a shield case. Therefore, the shield case is large-sized so that the charger is large-sized.
Further, when the saw-toothed electrodes are aged and cleaned at ends thereof, these electrode ends are rapidly worn and deteriorated when these electrode ends are steep. Accordingly, no saw-toothed electrodes can be practically used in such a case.